[Mobile-Portable] Unwanted Antenna Coupling between close antennas

[N1KHB at aol.com]

Hi Ray,
   I think you've been rather thorough asking the  right questions of your 
own approach as well as doing what I consider to be the  right basic tests 
at least in the practical shoot-from-the-hip sense which is my  usual 
standard and style too. :-) One point however is that it  appears you've tested at 
more or less arbitrary frequencies. I might  recommend that you do the same 
tests with harmonically related frequencies. One  example I can cite is a 
situation that exists even to this day. Our local 2  M repeater has an output 
of 145.29 MHz. There's a low power transmitter  operating in the AM 
broadcast band used by our DOT to issue driver alert  messages that the state 
considers useful to drivers by listening to their car AM  receiver. I don't recall 
the frequency as I have it set on a push button memory  position. That AM 
transmitter (I don't know that it's even licensed) easily gets  into my 2 
meter radio when set at 145.29 and with a quarter mile or so, and it's  
intelligible no less! Who'd have thought such a problem would exist considering  
the two different operating modes and with such a high harmonic content? The  
main point is that a test on an arbitrary frequency might well yield 
different  results than on harmonically related frequencies. Maybe the  undesired 
signal gets into the front end, maybe it's getting into the IF. I  don't know 
and don't especially care since the result is still the same. Give it  a 
try and let us or at least me know of the outcome.
   I'm also asking in a rhetorical manner what the  reason is that the 
antennas need to be so close to each other. And finally, what  questions are you 
trying to answer of the HF antenna installation method? 
  HTH.
 
Best,
Sonny N1KHB
 
 
 
 
In a message dated 7/28/2016 6:50:50 P.M. Eastern Daylight Time,  
w4byg at att.net writes:

Gentlemen (and Gentle Ladies),
I recently read W8JI's .pdf on  "Unwanted Antenna Coupling" and found it 
very interesting.  My  immediate area of concern is on my new mobile 
installation.

I have  2 antennas mag mounted on the top of my 2016 RAM 1500 crew cab.  
One  is a Comet HP-32 dual band U-V antenna (running about 50 watts) and 
the  other is a typical base loaded 3' CB antenna (typically running 
about 4  watts).  They are about 1 foot apart, thus worthy of  
consideration.

My basic tests for any dangerous antenna coupling from  one transmitter 
interfering or damaging an adjacent receiver, has been to  turn one 
receiver on, listening to the noise level and keying the other  
transmitter at low power, listening for any desensing or change in noise  
level.  I  then increase the transmit power in steps, as seems  
acceptable.  Hearing little or none, I reverse the process and listen  to 
the other receiver while keying the other transmitter.

I then  tune in a station on the air and use the same process to listen 
for any  cross modulation or distortion, on the audio.  I also listen for 
any  "key clicks" when keying.

I have used this procedure on a previous  mobile installation where I ran 
the same 2 radios above, plus a 500 watt  HF rig usually on 20 or 40 
meters, with the 8' center loaded whip within  about 2 feet of the other 
two and found little to no affect on the  adjacent receivers.

(I've yet to decide on an acceptable way to mount  the HF antenna on the 
new truck).

The most change I observe above  has been a very slight noise desense of 
the CB receiver from the 2 meter  transceiver and in the previous 
installation a little from the HP HF rig  on 20 meters.  But the desense 
seemed to be very slight, maybe 1 or 2  db and thus tolerable.  None of 
the "S" meters showed any RF at any  time during the tests.

I use a similar procedure in checking the auto  AM-FM radio for RF 
susceptability from my radios.

It would seem the  difference in frequency spectrum of the different 
rigs, plus whatever  bandpass filtering they normally have built into to 
the front ends are  basically sufficient to protect the active stages, 
even with the close  vertical antenna spacings in my mobile installations.

Anyone have any  comments or otherwise suggestions?
Regards,
Ray, W4BYG

-- 
I'm  no longer young enough to know  everything!

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